Unlock Seung Mina
In chapter 6 of weapon master mode, a palace icon will be in the middle. Fight Seung Mina there, win, and unlock the Korean princess for play in all modes.

Unlock Characters in Weapon Master Mode
Play through Weapon Master mode to unlock characters, modes and stages. As you beat a character in Weapon Master Mode, that character will be unlocked for use in other modes. Unlockable characters include:

Cervantes
Charade
Lizard Man
Seung Mena
Yoshimitsu

Unlock Sophitia
In weapon mode, keep hitting Sophita's little sister to unlock a playable version of her character.